Centrally fed antenna system and method for optimizing such an antenna system

ABSTRACT

A centrally fed reflector antenna system has an effective reflector surface shaped so that the maximum of the copolar far field lies on the illuminated coverage area corresponding to the far field requirements, and the minimum of the copolar near field lies at the feed system, e.g. at the aperture of a feed horn.

The invention concerns a centrally fed antenna system and a process tooptimize it.

Centrally fed antenna systems usually have a single reflector and a feedsystem, although double reflector systems are known where the feedsystem irradiates a subreflector that itself irradiates a mainreflector. In the following, only a single reflector antenna system willbe discussed; however, the designs can also be used for doublereflectors.

In comparison to antennas with a single reflector and offset feedsystem, centrally fed antenna systems with a single reflector are morecompact. In regard to the electromagnetic properties, a centrally-fedantenna does not have offset cross-polarization and hence generates lesscross-polarization than an antenna system with a single reflector and anoffset feed system. However, centrally-fed antenna systems have twosubstantial disadvantages in regard to electromagnetic properties:First, the electromagnetic field sent by the reflector is shaded by thefeed system, the supports for the feed system, and the feed cable;second, this electromagnetic field affects the feed system. The shadingbasically influences the copolar polar antenna pattern. It produces aripple in the pattern in the main beam direction and changes the levelof the side lobes. Additional cross-polarization arises for circularpolarized, centrally fed antennas. The effect on the feed system fromthe near field reflected by the reflector basically influences thecross-polarized antenna pattern and the reflection factor of the overallsystem.

The shading can be reduced by making the parts of the antenna system inthe near field (that is, the supports, feed system and cable) astransparent as possible for the electromagnetic field. In addition,electrically conductive sheathing can reduce additional scatter in thenear field and hence noise in the far field.

Dispersion or scatter bodies such as small cones that are placed in thecentre of the reflector can reduce the effect of the near field on thefeed system. The scatter bodies are shaped so that the stray field thatproceeds from them and the near field reflected by the reflectordestructively overlap at the feed system so that a zero area isgenerated at this location. This stray field of course also influencesthe far field as well.

The invention is based on the problem of modifying a centrally fedantenna system so that the effect of the shading and the reaction of thefeed system are clearly reduced. In addition, a procedure will bepresented to attain this.

The features of patent claim 1 solve these problems regarding acentrally fed antenna system. In regard to the procedure, these problemsare solved by the features of the additional independent patent claims.

Basically, the entire effective reflector surface is shaped so that themaximum of the copolar far field lies on the irradiated coverage areacorresponding to the requirements of the far field, and the minimum ofthe copolar near field lies at the feed system, e.g. at the aperture ofa horn.

The actual shape of the effective surface of the reflector system isdetermined on a computer with a software program. First the surface ofthe reflector is calculated using a program according to therequirements of the copolar far field. The influences of the effectbetween the reflector surface and feed system can be initially ignored.There exists such a prior-art program and is generally termed a POprogram, i.e., physical optics (see for example Stig Busk Sorensen:Manual for POS, Physical Optics Single Reflector Shaping Program; TICRAEngineering Consultants, Copenhagen, Denmark, June, 1995). A computermodel of an antenna system adapted to the requirements of the copolarfar field is obtained.

This computer model is then optimized with an optimization program thatis used basically for the entire effective reflector surface so that theeffects of the near field on the feed system are essentially reduced tonothing without basically changing the properties of the copolar farfield.

Such a procedure that optimizes the entire effective antenna surfacesubstantially improves the reflection factor of the entire system andthe copolarization and cross-polarization properties.

The invention will be further explained using an exemplary embodimentwith reference to the drawing. Shown are:

FIG. 1 a schematic perspective view of a centrally fed antenna with ahorn as the feed system and a single reflector whose surface is shapedaccording to the invention;

FIG. 2 a schematic perspective view of the deviation of the surfaceshape of the reflector shaped according to the invention from aconventional parabolic reflector;

FIG. 3 a representation of the reflection factor of the overall systemfor a reference system with a parabolic reflector for the polarizationin the X direction and for an antenna system according to the inventionfor the polarization in the X and Y directions;

FIGS. 4a-4 d comparisons of the antenna patterns in the elevation andazimuth above the coverage area in copolarization and cross-polarizationfor a reference system and an antenna system according to the invention.

FIG. 1 shows a centrally fed antenna system 1 with a single reflector 2and a feed system (a horn 3 in this case), where the horn is held byfour supports 4 in the middle above the reflector 2 and is fed by acable 5.

The reflector 2 is a parabolic reflector that is designed according toconventional methods so that a desired coverage area (FIG. 4) issufficiently illuminated. The antenna system 1 is e.g. used on acommunications satellite so that the coverage area is a specific area onthe earth's surface.

To reduce the attenuation of the far field by the horn, the supports andcable, the supports 4 are designed as braces with a honeycomb structuremade of fiber-reinforced plastic. Aramide fibers are preferably used.The horn 3 is generally covered with a reflective foil (such asaluminium foil) which in particular serves to prevent reflections of thenear field on sharp edges, etc.

The surface of the parabolic reflector is first calculated with asoftware program so that the far field of the antenna system will coverthe desired coverage area. This is done e.g. with the above-cited POprogram.

Finally, a computer-supported optimization process is carried out usingan optimization program that essentially optimizes the entire reflectivesurface point for point to optimize the requirements for the near fieldand those in the far field. The requirements for the near field areessentially that the surface be shaped so that a zero area arises at theaperture of the horn in the copolar near field, and a maximum isgenerated on the coverage area in the copolar far field.

FIG. 2 contrasts the attained deviations of the optimized reflectorsurface with the preshaped reflector surface. The data concern anantenna reflector with a diameter of 100 cm and a spacing of the hornaperture above the centre of the parabolic reflector of 40 cm. Thefrequency band for this antenna is 5.8 to 6.4 GHz with dual linearpolarization. The deviations in FIG. 2 of the optimized reflector 2 fromthe preshaped parabolic shape are between −1.74 mm and +4.41 mm.

FIG. 3 shows the reflection factor of the overall system in comparisonto the reference system with a preshaped parabolic reflector in afrequency band of 5.6 to 6.5 GHz. 7 indicates the curve of the referencesystem in copolarization; 8 is the corresponding curve for the optimizedantenna system according to FIGS. 1 and 2. One can see that the valuesare clearly improved. 9 shows the cross-polarization curve for theantenna system according to the invention. The average amplitude for theoverall system is approximately 22 dB.

FIG. 4 shows antenna patterns over the coverage area for the referencesystem with a parabolic reflector, and for the antenna system accordingto the invention. FIGS. 4a and 4 b show the copolar antenna patterns forthe reference system and the system according to the invention. Thelines are given the respective dB values. In the reference system inFIG. 4a, one can see an area 10 in the middle of the coverage areadelimited by a line and is assigned 24 dB. Such an area does not existin FIG. 4b in the antenna system according to the invention. The overallcoverage system of the antenna system according to the invention isapproximately delimited by an area of 24 dB. By optimizing the entiresurface of the antenna reflector according to the invention, the copolarfar field can be given a better design. The disturbance in the copolarfield due to the loss from the horn, braces and the cable is greatlyreduced by the antenna system according to the invention.

FIG. 4c shows the cross-polarization antenna pattern of the referencesystem. FIG. 4d shows the pattern of the antenna system according to theinvention. One can clearly see that the properties of the antenna aresubstantially improved, i.e., the optimization of the overall reflectorsurface reduces the influence of the near field on the feed system.

The overall system is generally improved enough that the disturbancefrom the attenuation and subsequent effect on the feed system areapproximately that of an equivalent interfering transmitter of more than−30 dB.

The table at the conclusion of the description shows the values for themaximum overall reflection factor, the minimum gain at the edge of theilluminated coverage area, the minimum gain in the coverage are in afrequency band of 5.854 to 6.298 GHz, the maximum cross-polarization inthe overall coverage area and the minimum cross-polarizationdiscrimination XPD, i.e., a point-for-point correlation between thecopolarization and cross-polarization in the entire illuminated coveragearea also in a frequency band of 5.854 to 6.298. This is for a parabolicantenna serving as a reference, a parabolic antenna with a centralscattering body, and an antenna system whose entire reflector surfacewas reshaped according to the invention.

One can see that the antenna cross-polarization properties from theeffect of the near field on the feed system can be improved more byreshaping the overall reflector surface than by using scattering bodies.The antenna copolarization properties at the edge of the coverage areaare better with an optimized reflector surface according to theinvention than when scattering bodies are used. The scatter bodiesdisturb the entire field that was originally designed for therequirements of copolarization. In contrast, the reshaped surface of thereflector according to the invention is an optimum compromise betweenthe copolar antenna properties and the reduction of the effect on thefeed system.

Overall, the reformation of the reflector surface yields betterelectrical properties than the use of scattering bodies.

Although the above antenna system is optimized with a single reflector,of course antenna systems with double reflectors can be optimized aswell, i.e., a subreflector and a main reflector according to theinvention. The subreflector illuminated by the feed system is optimizedover its entire surface to minimize the effect on the feed system andoptimally illuminate the main reflector. Then the main reflector isoptimized so that the maximum of the copolarization on the coverage areais maximized, and the effect on the subreflector is minimized.

In all the procedures according to the invention, the optimizationcorresponds well with the initial analysis, i.e., the measuredproperties of the antenna system correspond very well with thecalculated properties. The procedure offers a highly effective tool forconstructing antenna systems without complicated and exhaustiveexperiments.

TABLE Original Original reflector sur- reflector sur- face with facewithout plate 90 Reshaped scatter mm in dia. reflector bodies Pos. 356.4surface Pol. Pol. Pol. Pol. Pol. Pol. X Y X Y X Y Measurement: maximum−15.0 −22.0 −21.2 −23.9 overall reflection factor dB dB dB dB between5.850 and 6.425 GHz Measurement: minimum 23.11 23.69 22.95 23.10 23.8623.73 gain at the edge of the dBi dBi dBi dBi dBi dBi illumination areabetween 5.854 and 6.298 GHz (without cable losses) Measurement: minimum23.17 23.58 23.00 23.09 23.96 23.85 gain within the illumina- dBi dBidBi dBi dBi dBi tion area between 5.854 and 6.928 GHz (without cablelosses) Measurement: maximum +3.64 +4.76 −1.11 −0.29 −4.37 −5.32cross-polarization on the dBi dBi dBi dBi dBi dBi overall illuminationarea between 5.854 and 6.298 GHz (without cable losses) Measurement:maximum 21.87 19.90 26.06 24.80 29.44 29.82 XPD on the overall illumi-dB dB dB dB dB dB nation area between 5.854 and 6.298 GHz (without cablelosses)

What is claimed is:
 1. An antenna system comprising: a feed system; anda reflector system illuminating a coverage area which reflector systemhas at least one parabolic reflector with a structured surface; whereina reflector surface of the parabolic reflector has peaks and valleysthat are disposed alternately in a radial direction, and that are atleast partially overlapped in a peripheral direction with other peaksand valleys; and the entire structure of the reflector surface has peaksand valleys, with a maximum of a copolar far field lying on the coveragearea, and a minimum of a copolar near field lying at the feed system. 2.The antenna system according to claim 1, wherein the reflector surfaceis shaped so that the copolar far field is substantially unchanged whenthe near field is optimized to reduce the effect on the feed system. 3.The antenna system according to claim 1, wherein the feed systemcomprises a horn with a small aperture diameter.
 4. The antenna systemaccording to claim 1, wherein the feed system has supporting braces thathave a honeycomb structure of fiber-reinforced material.
 5. The antennasystem according to claim 1, wherein: the reflector system comprises amain reflector and a subreflector; and surfaces of the main reflectorand the subreflector have peaks and valleys.
 6. A process for providingan optimized centrally-fed antenna system having a feed system and areflector system with at least one reflector illuminating a coveragesurface, said process comprising: determining a parabolic surface for atleast one reflector; calculating a far field of the antenna system witha first computer program; and pre-shaping substantially the entirereflector surface of the at least one reflector with a second computerprogram to form at least partially peripherally extending peaks andvalleys disposed sequentially in a radial direction, such that a minimumof a copolar near field is generated in the area of the feed system, anda maximum of the copolar far field lies on the coverage surface.
 7. Aprocedure according to claim 6, having a main reflector and asubreflector, wherein the subreflector surface is first optimized, andthen the main reflector surface of the reflector system is optimized. 8.An antenna system comprising: a feed system; and a reflector systemilluminating a coverage area, which reflector system has at least onesubstantially parabolic reflector with a structured surface; wherein areflector surface of the substantially parabolic reflector has at leastpartially peripherally extending peaks and valleys that are disposedsequentially in a radial direction, and that are at least partiallyoverlapped in a peripheral direction with other peaks and valleys; andsubstantially the entire structure of the reflector surface has peaksand valleys, such that a maximum of a copolar far field lies on thecoverage area, and a minimum of a copolar near field lies at the feedsystem.
 9. An antenna system comprising: a signal feed element; and areflector element disposed to reflect signals from the signal feedelement, reflected signals from the reflector element illuminating acoverage area; wherein the reflector element has a reflecting surfacethat has a substantially parabolic contour, and includes deviations fromsaid parabolic contour, which deviations form a pattern of peaks andvalleys in said reflecting surface such that a minimum of a copolar nearfield generated by said antenna system lies in substantial proximity tosaid signal feed element.
 10. An antenna system according to claim 9,wherein said pattern of peaks and valleys is such that a maximum of acopolar far field generated by said antenna system lies substantially onthe coverage area.
 11. The antenna system according to claim 9, whereina longitudinal direction of said pattern of peaks and valleys extendssubstantially circumferentially about a central axis of said reflectingsurface.
 12. The antenna system according to claim 10, wherein alongitudinal direction of said pattern of peaks and valleys extendssubstantially circumferentially about a central axis of said reflectingsurface.
 13. The antenna system according to claim 9, wherein saidpattern of peaks and valleys is such that a radial cross section of saidreflecting surface contains a series of sequential positive and negativevariations.
 14. The antenna system according to claim 10, wherein saidpattern of peaks and valleys is such that a radial cross section of saidreflecting surface contains a series of sequential positive and negativevariations.
 15. The antenna system according to claim 9, wherein saidpattern of peaks and valleys includes a plurality of at least partiallycircumferentially extending peaks which alternate with at leastpartially circumferentially extending valleys.
 16. The antenna systemaccording to claim 10, wherein said pattern of peaks and valleysincludes a plurality of at least partially circumferentially extendingpeaks which alternate with at least partially circumferentiallyextending valleys.
 17. The antenna system of claim 9, wherein saidsignal feed element comprises a centrally disposed signal feed element.18. The antenna system of claim 10, wherein said signal feed elementcomprises a centrally disposed signal feed element.